The present invention relates generally to rotatable members that are able to achieve balanced conditions throughout a range of rotational speeds. The present invention also relates to methods and systems for dynamically balancing rotatable members through the continual determination of out of balance forces and motion to thereby take corresponding counter balancing action. The present invention additionally relates to methods and systems in which inertial masses are actively shifted within a body rotating on a shaft in order to cancel rotational imbalances associated with the shaft and bodies co-rotating thereon. The present invention additionally relates to methods and system that extract balance information from measured signals that are used in assessing the balance condition and determining the course of dybnamic balance control.
When rotatable objects are not in perfect balance, nonsymmetrical mass distribution creates out-of-balance forces because of the centrifugal forces that result from rotation of the object. This mass unbalance leads to machine vibrations that are synchronous with the rotational speed. These vibrations can lead to excessive wear and unacceptable levels of noise.
It is a common practice to balance a rotatable body by adjusting a distribution of moveable, inertial masses attached to the body. In general, this resulting state of balance may remain until there is a disturbance to the system. A tire, for instance, can be balanced once by applying weights to it and the tire will remain balanced until it hits a very big bump or the weights are removed. However, certain types of bodies that have been balanced in this manner will generally remain in balance only for a limited range of rotational velocities. One such body is a centrifuge for fluid extraction, which can change the degree of balance as speed is increased and more fluid is extracted.
Many machines are configured as freestanding spring mass systems in which different components thereof pass through resonance ranges during which the machine may become out of balance. Additionally, such machines may include a rotating body loosely coupled to the end of a flexible shaft rather than fixed to the shaft as in the case of a tire. Thus, moments about a bearing shaft may also be created merely by the weight of the shaft. A flexible shaft rotating at speeds above half of its first critical speed can generally assume significant deformations, which add to the imbalance. This often poses problems in the operation of large turbines and turbo generators.
Machines of this kind usually operate above their first critical speed. As a consequence, machines that are initially balanced at relatively low speeds may tend to vibrate excessively as they approach full operating speed. Additionally, if one balances to an acceptable level rather than to a perfect condition (which is difficult to measure), the small remaining xe2x80x9cout-of-balancexe2x80x9d will progressively apply greater force as the speed increases. This increase in force is due to the fact that F is proportional to rxcfx892, (F is the out of balance force and r is the radius of the rotating body and xcfx89 is its rotational speed).
The mass unbalance distributed along the length of a rotating body gives rise to a rotating force vector at the bearings that support the body. In general, the force vectors at respective bearings are not in phase. The rotating force vector may be opposed by a rotating reaction force, which can be transmitted to the bearing supports as noise and vibration. The purpose of active, dynamic balancing is to shift an inertial mass to the appropriate radial eccentricity and angular position for canceling the net mass unbalance. At the appropriate radial and angular distribution, the inertial mass can generate a rotating centrifugal force vector equal in magnitude and phase to the reaction force referred to above. Although rotatable objects find use in many different applications, one particular application is a rotating drum of a washing machine.
Many different types of balancing schemes are known to those skilled in the art. U.S. Pat. No. 5,561,993, which issued to Elgersma et al. on Oct. 22, 1996, and is incorporated herein by reference, discloses a self-balancing rotatable apparatus. Elgersma et al. disclosed a method and system for measuring forces and motion via accelerations at various locations in a system. The forces and moments were balanced through the use of a matrix manipulation technique for determining appropriate counterbalance forces located at two axial positions of the rotatable member. The method and system described in Elgersma et al. accounted for possible accelerations of a machine, such as a clothes washing machine, which could not otherwise be accomplished if the motion of the machine were not measured. Such a method and system was operable in association with machines that are not rigidly attached to immovable objects, such as concrete floors. The algorithm disclosed by Elgersma et al. permitted counterbalance forces to be calculated even when the rotating system (such as a washing machine), was located on a flexible or mobile floor structure combined with carpet and padding between the washing machine and a rigid support structure.
U.S. Pat. No. 5,561,993 thus described a dynamic balance control algorithm for balancing a centrifuge for fluid extraction. To accomplish balance control, sensor signals are filtered through correlation for balance measurements (i.e., magnitude and phase at the frequency component associated with the rotational speed) whose responses to balancing control actions are modeled and utilized to determine control actions that drive the system toward a balanced state. Such a system is generally time variant, such that the control models utilized therein may need to be routinely updated based on the measured response to a previous control action, which is a variation of perturbation theory, well known in the art.
In developing enhancements to the control algorithm explained in U.S. Pat. No. 5,561,993, it was observed that balance measurements were corrupted by low frequency modulations, especially noted at higher rotational speeds. An original filter implementation associated with U.S. Pat. No. 5,561,993 applied a fixed-number-of-revolutions rectangular window to the data, resulting in a decreased time span of data and an associated widening of the filter bandwidth as rotational speed was increased. This implementation resulted in poor isolation of the desired signal component at higher rotational speeds, thereby leading to corrupt balance measurements.
Assessing the balance state and creation of control models using corrupt balance measurements may lead to inadequate control actions. Based on the foregoing, it can be appreciated that this balance measurement related issue could lead to a limited degree of achievable balance, lengthy balancing times and the inability to obtain maximum spin speeds in centrifuge environments, such as, for example, a washing machine. Improved balance control and times can be achieved by addressing these issues. The invention described herein can overcome this balance measurement issue through the use of a fixed time span, and fixed bandwidth, correlation method and system in the correlation-based filtering of sensor signals.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
In accordance with addressing the shortcomings of the prior art, it is one aspect of the present invention to provide methods and systems in which rotatable members can achieve balanced conditions throughout a range of rotational speeds.
It is another aspect of the present invention to provide methods and systems for dynamically balancing rotatable members through the continual determination of out of balance forces and motion to thereby take corresponding counter balancing action.
It is yet another aspect of the present invention to provide methods and systems for measuring the dynamics of rotating systems and devices thereof in order to make corrections necessary to placing such systems or devices in a balanced condition.
It is still another aspect of the present invention to provide methods and systems for extracting balance measurements from sensor signals using correlation techniques, thereby determining the dynamics of rotating systems such that a balanced state is achieved in a shorter period of time.
In accordance with various aspects of the present invention, methods and systems are disclosed herein for dynamically balancing a rotating system utilizing fixed-bandwidth correlation techniques to define balance measurements from sensor signal data associated with the rotating system over its full range of operational speeds. The sensor signal data contains information indicative of the dynamics of the rotating system. This information is extracted through correlation techniques to provide balance measurements in the form of magnitude and phase of the frequency component associated with the rotational speed. The balance measurements can then be used to assess the balance state of the rotating system and update control models so as to determine the proper course for balancing corrections necessary to dynamically place the rotating system in a balanced state.
Initially, raw sensor data is low-pass filtered and then sampled. To simplify later computation, a fixed number of samples per revolution may be collected independent of the speed of rotation. This may be accomplished through variable-rate sampling or fixed-rate over-sampling and decimation strategies. A correlation operation is performed on a window of the sampled data to filter the desired balance measurements. The windowed data is an integer number of revolutions greater than or equal to 1. The time span of the windowed data is generally inversely proportional to the bandwidth of the filtering operation.
The present invention changes a correlation-based digital filter in that the time span of data no longer represents a fixed number of revolutions across all rotational speeds, with a decreasing time span as rotational speed increases. For the present invention, a fixed time span associated with the window of the data may be selected such that it is generally of sufficient size to establish a filter bandwidth that extracts the signal component of interest and attenuates lower frequency modulations associated with the rotating system. Thus, as rotational speed is increased, the number of revolutions of data in the window is increased. This can be based on a fixed reference window-time adjusted upward as a function of rotational speed to represent a minimum number of full revolutions of data samples extending beyond the reference time. The reference time may be selected larger than the period of low frequency modulations to be eliminated.
Additionally, several fixed time span windows may be designated, wherein each may be utilized for a range of rotational speeds for which it provides adequately small filtering bandwidth. Employing window functions multipliers different from the rectangular function, on the designated time span of data, will further assist to drastically reduce filter side-band ripple.